Field of the Disclosure
The present disclosure relates to a nonaqueous electrolyte for a lithium secondary battery, which includes a hetero polycyclic compound, and a lithium secondary battery using the same.
Description of the Related Art
In recent years, there has been increasing interest in energy storage technologies. As the application fields of energy storage technologies have been extended to mobile phones, camcorders, notebook computers and even electric cars, there has been a growing demand for high energy-density batteries as power sources for such electronic devices. In response to this demand, research on lithium secondary batteries is being actively undertaken.
Lithium secondary batteries developed in the early 1990's are made up of an anode of a carbon-based material capable of intercalating and deintercalating lithium ions, a cathode of lithium containing oxide, and a non-aqueous electrolyte containing a proper amount of lithium salts dissolved in a mixed organic solvent.
The average discharge voltage of the lithium secondary battery is about 3.6 to 3.7 V, which is higher than those of alkali batteries, nickel-cadmium batteries or the like. F or such a high operating voltage, an electrolytic composition electrochemically stable in a charge/discharge range of 0˜4.2 V is required. For this, a mixed solvent where a cyclic carbonate compound such as ethylene carbonate and propylene carbonate and a linear carbonate compound such as dimethyl carbonate, ethylmethyl carbonate and diethyl carbonate are appropriately mixed is used as a solvent of the electrolyte. A solute of the electrolyte commonly uses a lithium salt such as LiPF6, LiBF4, LiClO4 or the like, which serves as a lithium ion source in a battery and thus enables the lithium battery to operate.
At an initial charging stage of a lithium secondary battery, lithium ions emitting from a cathode active material such as a lithium metal oxide move to an anode active material such as graphite and are intercalated between layers of the anode active material. At this time, since lithium has strong reactivity, the electrolyte is reacted with carbon of the anode active material at the surface of the anode active material such as graphite, thereby generating compounds such as Li2CO3, Li2O and LiOH. These compounds form a kind of an SEI (Solid Electrolyte Interface) layer on the surface of the anode active material such as graphite.
The SEI layer plays role of an ion tunnel and allows only lithium ions to pass. Due to the ion tunnel effect, the SEI layer prevents organic solvent molecules, which move together with lithium ions in the electrolyte and have large molecular weight, from being intercalated between layers of the anode active material and thus destroying the anode structure. Therefore, since the contact between the electrolyte and the anode active material is prevented, the electrolyte is not dissolved, and the amount of lithium ions in the electrolyte is reversibly maintained, thereby ensuring stable charge/discharge.
However, during the SEI layer forming reaction, the battery thickness may increase at charging due to gases such as CO, CO2, CH4 and C2H6 generated by the dissolution of a carbonate-based solvent. In addition, when the battery is left alone in a fully-charged state at a high temperature, as time passes, the SEI layer may slowly collapse due to the increasing electrochemical energy and thermal energy, which cause side reaction to continuously occur between the exposed surface of the anode and the surrounding electrolyte. Due to the continuous generation of gas, the inner pressure of the battery increases, which results in the increase of thickness of the battery, causing problems in a device such as a cellular phone and a notebook using the battery. In other words, the high-temperature stability is inferior when the battery is left alone. In addition, the above problem caused by the increased inner pressure is more serious at a general lithium secondary battery containing a large amount of ethylene carbonate since the SEI layer is unstable. In addition, since ethylene carbonate has a high freezing point of 37 to 39° C., making it a solid state at room temperature, a lithium battery containing a large amount of ethylene carbonate due to low ion conductivity at a low temperature has bad low-temperature conductivity.
In order to solve this problem, many studies for changing a composition of solvent components in the carbonate organic solvent or changing the aspect of a SEI layer forming reaction by mixing a specific additive have been made. However, as know in the art, in the case of changing a solvent component or adding a specific compound to the electrolyte in order to improve the battery performance, even though some features are improved, other features are deteriorated instead.
Therefore, a nonaqueous electrolyte composition capable of providing a lithium secondary battery with good cycle characteristic, good low-temperature discharge characteristic and good high-temperature discharge characteristic as well as excellent high-rate charge/discharge characteristics should be urgently developed.